The present invention relates to a method of preparing silicone pressure sensitive adhesive compositions. More particularly the present invention relates to a method of making a silicone pressure sensitive adhesive composition comprising heating a mixture of a polydiorganosiloxane, a silicone resin copolymer, and an equilibration catalyst, and then adding an organic peroxide or azo compound to the resultant reaction product.
Silicone pressure sensitive adhesives (PSA's) are typically produced by either blending or condensing together a silicone resin and a silicone polymer. For example, Dexter et al. in U.S. Pat. No. 2,736,721 discloses a silicone pressure sensitive adhesive that is produced by mixing or condensing a silicone resin and a diorganopolysiloxane polymer having a viscosity of at least 1,000,000 centistokes.
Currie et al. in U.S. Pat. No. 2,814,601 discloses a silicone pressure sensitive adhesive that is produced by mixing together a silicone resin and a hydroxyl end-blocked diorganopolysiloxane fluid having a viscosity of from 5,000 to 1,000,000 centistokes. Currie et al. also discloses a permanent adhesive that is produced by adding a cross-linking agent and curing catalyst to the pressure sensitive adhesive composition.
Goodwin et al. in U.S. Pat. No. 2,857,356 discloses a silicone pressure sensitive adhesive that is produced by condensing together a silicone resin that is a cohydrolysis product of a trialkyl hydrolyzable silane and alkyl silicate with a linear organopolysiloxane fluid having a viscosity of greater than 200,000 centipoise.
Modic in U.S. Pat. No. 3,528,940 discloses compositions comprising the siloxane product of intercondensation at a temperature of from about 80.degree. C. to 150.degree. C. of a mixture of ingredients comprising parts of a silanol chain-stopped polydiorganosiloxane gum having a viscosity of at least about 10,000,000 centistokes at 25.degree. C., a benzene-soluble resinous copolymer containing R.sub.3 SiO.sub.0.5 units and SiO.sub.2 units where R is a monovalent hydrocarbon radical, and a finely divided silica having a surface area in excess of about 100 square meters per gram.
Horning in U.S. Pat. No. 3,929,704 discloses a pressure sensitive adhesive composition comprising a silicone resin, a silicone gum, and a curing agent therefor, the curing agent comprising 2,4dichlorobenzoyl peroxide in admixture with a plasticizer and an extender therefor, the extender comprising an alkylarylsiloxane or an alkylaralkylsiloxane fluid.
McInally et al. in U.S. Pat. No. 4,831,070 discloses a composition for forming elastomeric pressure sensitive adhesives comprising a mixture of a resin copolymer containing triorganosiloxy units and SiO.sub.4/2 units, a polydiorganosiloxane fluid endcapped with silicon-bonded hydroxy groups and having a viscosity of from about 50 to 300,000 centipoise at 25.degree. C., an organosilicon compound containing an average of more than two silicon-bonded alkoxy groups per molecule, and a condensation catalyst such as metal salts of carboxylic acids.
Sweet in U.S. Pat. No. 4,865,920 discloses a hot-melt silicone pressure sensitive adhesive composition comprising a silicone resin, silicone fluid, and an ester having the formula RC(O)OR' where R is a monovalent hydrocarbon radical having from 2 to 32 carbon atoms, and R' is a monovalent hydrocarbon radical having from 1 to 14 carbon atoms.
European Patent Application No. 0459292 discloses a composition suitable for use as a pressure sensitive adhesive, comprising a homogenous mixture of 50 to 95 parts by weight of a first pressure sensitive adhesive composition comprising (I) a solid, benzene soluble resin copolymer containing R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units where R is a monovalent hydrocarbon radical and (II) a hydroxyl end-blocked diorganopolysiloxane having a viscosity of about 100,000 to 500,000,000 centipoise at 25.degree. C., and 5 to 50 parts by weight of a second pressure sensitive adhesive composition comprising a resin copolymer as described for (I) above, and a hydroxyl end-blocked diorganopolysiloxane as described for (II) above. The EP'292 reference further discloses that the above composition can be prepared by heating the mixture at a temperature of 130.degree. C. in an organic solvent and in the presence of a base catalyst such as alkali metal hydroxides or aminofunctional silanes. The EP'292 reference further discloses that acids for neutralizing the condensed mixture include mineral acids such as hydrochloric acid or phosphoric acid, and organic acids such as acetic acid, or acetyl chloride. The EP'292 reference further discloses that, if desired, a catalyst may be employed to assist in the curing, examples of which include benzoyl peroxide, dichlorobenzoyl peroxide, and di-t-butyl peroxide.
Hamada et al. in U.S. Pat. No. 5,100,976 discloses a silicone pressure sensitive adhesive composition comprising a polydiorganosiloxane, an organopolysiloxane consisting essentially of R.sub.3 SiO.sub.1/2 units and (R.sup.2 O).sub.a SiO.sub.(4-a)/2 units where R is a monovalent hydrocarbon radical, and an organopolysiloxane consisting essentially of R.sub.3 SiO.sub.1/2 units and (R.sup.2 O).sub.a SiO.sub.(4-a)/2 units where R is as defined above in a molar ratio of 1:1 to 1:7. Hamada et al. further discloses that their compositions can further comprise a curing agent such as an organic peroxide.
U.S. Pat. No. 5,248,739 to Schmidt et al. discloses a silicone pressure sensitive adhesive that exhibits low adhesion to low energy substrates. The adhesive is produced by mixing together a silicone resin copolymer containing R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units where R is a monovalent hydrocarbon radical and having a number average molecular weight of about 950 to 1,600 and a polydiorganosiloxane polymer. Schmidt et al. further discloses that these compositions can further comprise an organic peroxide as a crosslinking agent when the polydiorganosiloxane polymer does not contain unsaturated groups.
However, nowhere does Schmidt et al. disclose a method of making silicone pressure sensitive adhesives whereby a mixture of a polydiorganosiloxane, a silicone resin, and an equilibration catalyst are first heated to form a reaction product, and then an organic peroxide or azo compound is added to the reaction product. Schmidt et al. also do not disclose that a higher molecular weight resin can be employed is such a method.